An excellent question! Jewish (and Jesus') Purgatory is compared to Gehenna because in ancient rabbinic theology Jews with mediocre spirituality are NOT PERMANENTLY confined to Gehenna. For example, in one ancient rabbinic tradition, such Jews are released from Gehenna in a year! Thus, the fires of Gehenna are figurative, purifying, and educational.
This point is thrillingly important because Gehenna is Jesus' preferred term for Hell and because He elaborates His Gehenna concept with the image of Hell as a debtor's prison, from which ultimate relase is possible by paying off sin's debt (See e. g. Matthew 18:34). By implication, God's love never permanently abandons anyone after death, because in some unspecified ways the "debt" of Hell-bound sinners can be paid off. This insight removes the stigma of a vindictive unloving God of wrath and transforms Gehenna into an educational system. In this respect, it is important to note that the Hebrew (olam) and Greek (aionios) words often translated "eternal" don't actually mean that; rather, they mean for an indefinite or age-long period of time.